An electric power steering apparatus, that an assist steering device of a vehicle by means of a rotational torque of a motor, applies a driving force of the motor as an assist force to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. Such a conventional power steering apparatus performs a feedback control of motor current in order to generate steering assist torque (steering assist force) accurately. The feedback control adjusts the voltage supplied to the motor so that a difference between a current command value and a detected motor current value becomes small or zero, and the adjustment of the voltage applied to the motor is generally performed by the adjustment of a duty ratio of PWM (Pulse Width Modulation) control.
A general configuration of an electric power steering apparatus will be described with reference to FIG. 1. A column shaft (steering shaft) 2 coupled to a steering wheel (handle) 1 is coupled to tie rods 6 of steered wheels through reduction gears 3, universal joints 4a, 4b, and a rack-and-pinion mechanism 5. The column shaft 2 is provided with a torque sensor 10 for detecting the steering torque of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is coupled to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit 100 for controlling the electric power steering apparatus from a battery 14, an ignition key signal is also inputted the control unit 100 through an ignition switch 11, and the control unit 100 calculates a current command value of the steering assist command on the basis of a steering torque T detected by the torque sensor 10 and a velocity V detected by a velocity sensor 12, and controls a current to be supplied to the motor 20 on the basis of a current control value E which is calculated by performing compensation and so on to the current command value.
The control unit 100 mainly comprises a CPU (or an MPU or an MCU), and general functions performed by a program within the CPU are shown in FIG. 2.
The functions and operations of the control unit 100 will be described with reference to FIG. 2. The steering torque T detected by the torque sensor 10 and the velocity V from the velocity sensor 12 are inputted to a current command value calculating section 101, and a current command value Iref1 is calculated based on the steering torque T and the velocity V in the current command value calculating section 101. The steering torque T, a motor angular speed ω and a motor angular acceleration ω* are inputted to a torque compensator 110, and a torque compensation value Cm is calculated. The current command value Iref1 calculated by the current command value calculating section 101 is phase-compensated by a phase compensator 102 to increase the stability of the steering system. A current command value Iref2 phase-compensated by the phase compensator 102 and the torque compensation value Cm are inputted to an adder 103, and an added result of the adder 103 is outputted as a current command value Iref3. The current command value Iref3 is inputted to a maximum current limiter 104, the maximum current limiter 104 limits the maximum of the current and outputs a limited current command value Iref4.
The current command value Iref4 is inputted to a subtractor 105, and a deviation (Iref4-i) between the current command value Iref4 and a feedback current value i is calculated at the subtractor 105. The deviation (Iref4-i) is controlled by a PI-controller (proportional integral controller) 106, and then is inputted to a PWM-controller 107 in which a duty adjustment is performed. The PWM-controller 107 outputs the current control value E to an inverter 108, and the inverter 108 controls the motor 20 on the basis of the current control value E. The motor current value of the motor 20 is detected by a motor detection means 21, and is inputted to the subtractor 105 to be feed-backed.
A rotation sensor 22 such as a resolver is mounted on the motor 20, a motor rotation signal θ from the rotation sensor 22 is inputted to a motor angular speed calculating section 23, and the motor angular speed calculating section 23 calculates a motor angular speed ω which is a rotation angular speed of the motor 20. Furthermore, the motor angular speed ω is inputted to a motor angular acceleration calculating section 24, and the motor angular acceleration calculating section 24 calculates and outputs a motor angular acceleration ω* which is a rotation angular acceleration of the motor 20.
The torque compensator 110 comprises, for instance, an SAT (Self Aligning Torque) estimating section 111, a differential compensator 112, a convergence controller 113 and an inertia compensator 114 and so on. The SAT estimating section 111 inputs the steering torque T, estimates and outputs an SAT-value SATa. The differential compensator 112 differentiates the steering torque T and outputs a steering torque TA for increasing a response speed. The SAT-value SATa and the steering torque TA which is differential compensated are inputted to a subtractor 115, and the deviation between the steering torque TA and the SAT-value TA is outputted. The convergence controller 113 outputs a convergence control value Ga on the basis of the motor angular speed ω, the convergence control value Ga and the output of the subtractor 115 are added at an adder 116. In addition, the inertia compensator 114 outputs an inertia compensation value INa on the basis of the motor angular acceleration ω*, the inertia compensation value INa and the output of the adder 116 are added at an adder 117, the adder 117 outputs the compensation value Cm, and the compensation value Cm is inputted to the adder 103.
The SAT estimating section 111 stabilizes the vehicle behavior. The convergence controller 113 applies the brake to a swing operation of the steering wheel in order to improve the yaw-convergence characteristics of the vehicle. The inertia compensator 114 removes the torque that accelerates or decelerates the motor inertia from the steering torque, and makes a steering feeling that is without an inertia feeling.
In such an electric power steering apparatus, the motor 20 generating a steering assist force is connected to the steering mechanism through the reduction mechanism (the reduction gears 3). Since the friction of the reduction mechanism is large, the steering feeling is deteriorated by the friction. Therefore, there is a need to comprise the friction compensating function to improve the steering feeling in the electric power steering apparatus.
For example, an electric power steering apparatus is disclosed in Japanese Patent No. 3082483 (Patent Document 1). The electric power steering apparatus comprises a friction compensator to calculate a friction compensation value on the basis of a rotation direction of a motor, and the friction compensation is performed by adding the compensation value to an assist command for controlling the motor, thus a good steering wheel returning characteristic can be obtained from any let-go steering angle, and also the steering feeling can be improved.
Moreover, in an electric power steering apparatus disclosed in Japanese Patent Application Laid-Open No. 2005-170257 (Patent Document 2), it presumes that a static friction torque is generated when it is determined that a motor is in before-rotation-start state, and a static friction compensation control is performed by providing a motor with the static friction torque compensation current which is a result of multiplying a differential value of a steering torque by a predetermined coefficient. Furthermore, it presumes that a dynamic friction torque is generated when it is determined that the motor is in after-rotation-start state, a dynamic friction compensation control is performed by providing the motor with the dynamic friction torque compensation current which is getting closer to a predetermined current value gradually until it is equal to the predetermined current value.